Japanese Patent Application Laid-Open No. 2006-135888 describes, for example, such a configuration that bidirectional data communication is enabled between two contact points to reduce the size of a system in which a data carrier device and a data carrier driving device execute bidirectional data communication.
In a data communication system described in the above Japanese Patent Application Laid-Open No. 2006-135888, the data carrier device includes a signal terminal for inputting a signal from the data carrier driving device and a reference voltage terminal for inputting a reference voltage. The signal input from the data carrier driving device to the signal terminal of the data carrier device includes a part whose duty ratio is different, and is a pulse signal whose pulse frequency is constant as a whole. The data carrier device includes a data communication circuit generating a clock signal and a data signal based on the input pulse signal, and executes all operations by using the input pulse signal as a source voltage. The data carrier device includes a switch circuit controlling whether or not to supply a constant current to output the data signal to the data carrier driving device through the signal terminal. The data carrier driving device includes a current detection circuit generating the data signal according to the current flowing to the signal terminal of the data carrier device.
The data carrier device generates the clock signal and the data signal to communicate data based on the pulse signal, which is input from the data carrier driving device, and includes a part whose duty ratio is different, and whose pulse frequency is constant as a whole. The data carrier driving device generates the data signal with the current detection circuit to communicate data, the current detection circuit detecting the current flowing to the signal terminal of the data carrier device, the current being controlled by the switch circuit for controlling whether or not to supply the constant current as the data signal of the data carrier device. Thereby, the bidirectional data communication can be executed between two contact points.
However, the above bidirectional data communication system includes the following problem. The data carrier device needs to execute all operations by using the pulse signal as a source voltage, the pulse signal being input from the data carrier driving device between two terminals of the signal terminal and the reference voltage terminal. The pulse signal from the data carrier driving device includes a part whose duty ratio is different. Thus, the effective means for steadily operating the data carrier device is to provide a constant voltage circuit in the data carrier device, the constant voltage circuit generating a constant voltage which becomes a source power for an internal circuit. At this time, it is necessary to cause any part of the pulse signal from the data carrier driving device not to be lower than a voltage generated by the constant voltage circuit. Thus, it is necessary to cause a voltage of a low voltage side of the pulse signal from the data carrier driving device to be higher than the voltage generated by the constant voltage circuit in the data carrier device.
Here, such a case will be considered that a voltage value detection circuit is, for example, provided to detect the pulse signal input from the data carrier driving device, the voltage value detection circuit detecting an absolute value of the input pulse signal voltage. In this case, it is necessary to consider the accuracy of the voltage value detection circuit, the accuracy of the constant voltage circuit in the data carrier device, the accuracy of an amplitude value of the input pulse signal voltage, and a voltage noise between the data carrier driving device and the data carrier device. Under such a consideration, it is desired to cause a voltage of a high voltage side of the pulse signal input from the data carrier driving device to be high enough.
However, when the data carrier device is produced in a semiconductor process, if the pulse signal voltage input from the data carrier driving device is higher, the withstand voltage required for the data carrier device becomes higher. Thus, the following is a problem: a component included in the data carrier device becomes larger, and the cost is increased due to the increase of a chip area.
The input signal from the data carrier driving device to the signal terminal of the data carrier device includes a part whose duty ratio is different, and is a pulse signal whose pulse frequency is constant as a whole. In this method, it is necessary to severely control the duty ratio of the input pulse signal, and to cause the data carrier driving device to be an expensive system configured with a microcomputer, and the like, so that this is also a problem to increase a cost of a total system.